1. Field of the Invention
The application relates to a method utilized in a wireless communication system, and more particularly, to a method of handling random access response in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
According to structure of the LTE system, a UE performs a random access procedure to derive a timing advance command which allows the UE to be synchronized with a serving base station on uplink timing for preventing signals transmitted from the UE from colliding with those sent from other UEs under the coverage of the base station. In addition, the timing advance command is transmitted through a random access response message of the random access procedure. Note that, random access procedure includes non-contention random access procedure and contention random access procedure, which shall be well known in the art, so it is not given herein. In detail, the network uses a medium access control protocol data unit (MAC PDU) to transmit the timing advance command to the UE. A MAC PDU consists of a MAC header and one or more MAC Random Access Responses (MAC RAR) and optionally padding. A MAC PDU header consists of one or more MAC PDU subheaders, and each subheader corresponds to a MAC RAR. Moreover, a MAC PDU subheader consists of the three header fields E/T/RAPID, where “RAPID” represents random access preamble identifier for identifying the transmitted random access preamble. The UE decodes the corresponding MAC RAR based on the RAPID. A MAC RAR consists of four fields R/Timing Advance Command/UL Grant/Temporary C-RNTI, where “R” represents a reserved bit, “Timing Advance Command” indicates an index value used to control the amount of timing adjustment that a UE has to apply, “UL Grant” indicates the resources to be used on the uplink, and “Temporary C-RNTI” indicates the temporary identity that is used by the UE during the contention based random access procedure. In addition, the allocation of the MAC PDU is based on RA-RNTI which is calculated by the UE and network according to the timing and RB allocation of the transmitted random access preamble.
Toward advanced high-speed wireless communication system, such as transmitting data in a higher peak data rate, LTE-Advanced system is standardized by the 3rd Generation Partnership Project (3GPP) as an enhancement of LTE system. LTE-Advanced system targets faster switching between power states, improves performance at the cell edge, and includes subjects, such as bandwidth extension, coordinated multipoint transmission/reception (COMP), uplink multiple input multiple output (MIMO), etc.
For bandwidth extension, carrier aggregation is introduced to the LTE-Advanced system for extension to wider bandwidth, where two or more component carriers are aggregated, for supporting wider transmission bandwidths (for example up to 100 MHz) and for spectrum aggregation. According to carrier aggregation capability, multiple component carriers are aggregated into overall wider bandwidth, where the UE can establish multiple links corresponding to the multiple component carriers for simultaneously receiving and/or transmitting. In carrier aggregation, the UE only has one RRC connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the NAS mobility information, and the security input. This cell is referred to as a primary cell (PCell). In the downlink, the component carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC). In addition, cells other than the PCell are named secondary cell (SCell).
Besides, in LTE-Advanced system, a control signaling (i.e. PDCCH assignment) for a component carrier may be transmitted on different component carrier, hereafter called cross carrier scheduling. For example, a PDCCH order for a random access preamble assignment of a non-contention random access procedure is given in one component carrier, and the random access preamble of the non-contention random access procedure is transmitted in another component carrier.
The applicant notices problems associated to a random access response of a random access procedure in the cross carrier scheduling. Please refer to FIG. 1, which is a schematic diagram of a random access response collision. In FIG. 1, the network assigns random access preambles respectively for UE1 and UE2 through the component carrier cc#1. The UE1 transmits the random access preamble to the network via the component carrier cc#1, and the UE2 transmits the random access preamble to the network via the component carrier cc#2. In other words, the network assigns the random access preamble to the UE2 with the cross carrier scheduling. However, the network may assign identical random access preambles to UE1 and UE2 at the same time. In this situation, the network responses to the two random access preambles only with one random access response in the component carrier cc#1 since the received random access preambles correspond to the same RAPID, and thereby causing random access response collision. Nevertheless, the UE1 and UE2 does not know the random access response is for itself or not, and both apply the timing advance command in the random access response for uplink timing adjustment. This may cause interference in component carrier cc#1 or component carrier cc#2 since the timing advance command is accurate only for the UE1 or UE2.
In order to avoid collision of the random access response collision, the network may avoid assign identical random access preamble on each component carrier at the same time. However, this may restrict random access preamble configuration, and limit random access preamble assignment flexibility on all component carriers.
Moreover, the network may assign a random access preamble for UE2 with cross carrier scheduling on the component carrier cc#1. The UE2 transmits the assigned random access preamble to the network via the component carrier cc#2. On the other hand, the UE1 randomly selects a random access preamble, and transmits the random access preamble to the network via the component carrier cc#1. However, the randomly selected random access preamble may be identical to the assigned random access preamble. The network responses to the two random access preambles only with one random access response in the component carrier cc#1 since the received random access preambles correspond to the same RAPID, and thereby causing random access response collision. Similarly, the UE1 and UE2 does not know the random access response is for itself or not, and both apply the timing advance command in the random access response for uplink timing adjustment. This may cause interference in component carrier cc#1 or component carrier cc#2 since the timing advance command is accurate only for the UE1 or UE2.
Further, due to the above specification, a UE may encounter scenarios, causing resource waste, described as follows.
In the first scenario, please refer to FIG. 2, which illustrates a schematic diagram of a random access response collision in cross carrier scheduling according to a first embodiment of the prior art. The network assigns a preamble code for a SCell with a PDCCH order on a PCell (namely cross carrier scheduling), and thus the UE performs a non contention based random access procedure on the SCell for uplink timing synchronization. Meanwhile, another UE performs a contention based random access procedure on the PCell for uplink timing synchronization. The UE performing the contention based random access procedure may select a preamble code, which is identical to the assigned preamble code for the SCell. When the network receives both of the random access preambles from the PCell and SCell, the network responses the two random access preambles with only one random access response since the two random access preambles are identical and correspond to the same RAPID. Assume that the random access response including a timing advance command is for the UE performing the non contention based random access procedure on the SCell, but both of the UEs apply the timing advance command in the random access response. In this situation, the non contention based random access procedure is performed successfully, and no impact for an uplink transmission on the SCell. However, the contention based random access procedure may not be successfully performed since the timing advance command is for the non contention based random access procedure. For example, the message 3 of the contention based random access procedure may encounter interference (i.e. collision) due to the timing advance command in the random access response is not accurate for the UE performing contention based random access procedure, and thus the network responses a HARQ NACK to the UE on the PCell. The UE performing contention based random access procedure may continuously transmit the message 3 to the network until a number of the received HARQ NACK achieves to a certain number, and then sends a new random access preamble on the PCell. As can be seen, the uplink synchronization operation is delayed due to retransmissions of the message 3, and the resources are wasted for the contention resolution of the contention based random access procedure.
In the second scenario, please refer to FIG. 3, which illustrates a schematic diagram of a random access response collision in cross carrier scheduling according to a second embodiment of the prior art. Unlike the first scenario, only non contention based preamble on the SCell is received by the network. In other words, the contention based preamble on the PCell is sent but not received by the network. However, the UE performing the contention based random access procedure may consider that the contention based preamble is received by the network due to the random access response is received and decoded on the PCell, and thereby performs the contention resolution of the contention based random access procedure. Thus, interference and resource waste are occurred during the contention based random access procedure. The detailed description can be referred from above, so it is omitted herein.
In the third scenario, please refer to FIG. 4, which illustrates a schematic diagram of a random access response collision in cross carrier scheduling according to a third embodiment of the prior art. Unlike the first scenario, only contention based preamble on the PCell is received by the network. In other words, the non contention based preamble on the SCell is sent but not received by the network. According to the prior art, the network response to the contention based preamble with a random access response at the PCell. However, the UE performing the non contention based random access procedure may consider that the random access response RAR is for the non contention based preamble, and thereby causing confusion of the random access response. In addition, the UE determines that the non contention based random access procedure is successful and applies the timing advance command in the random access response. Thus, interference and resource waste are occurred during the non contention based random access procedure on SCell. The detailed description can be referred from above, so it is omitted herein.